Traditionally, auditory evoked potentials are measured with extracorporeal devices using surface electrodes applied to the skin of the patient. Such equipment picks up far field auditory evoked potentials which mainly comprise:                early auditory evoked potentials (commonly referred to by the initials ABR). These potentials are normally made up of seven waves (“Les potentiels évoqués auditifs” [Auditory evoked potentials] by J. M. Guerit, 1993 published by Masson, Paris at pages 87 to 99 on evoked potentials), with the wave 1 being generated mainly by ramifications of the auditory nerve along the internal hair cells;        medium latency auditory evoked potentials (referred to by initials AMLR);        late auditory evoked potentials (referred to by the initials ALCR) and;        cognitive evoked potentials (referred to by the initials ACR).        
Such extracorporeal devices for picking up auditory potentials require a great deal of care and time to be put into operation. In addition, on safety grounds, it is essential to have complete metallic isolation between the machine and the person under test. The results obtained do not always accurately represent real auditory activity since standard equipment is specifically too sensitive to interfering electrical noise: stimulation artifacts; 50 Hz or 60 Hz from the electricity mains; high frequency interference; etc . . . Similarly, such equipment is distributed by non-auditory biological activity. Furthermore, result reproducibility is degraded by fluctuations in the resistivity of the skin and by impedance drift due in particular to moving electrodes which can shift during a test, and which above all can be positioned differently from one examination to another. In order to restrict these drawback at least in part, it is possible to make use of anesthetics, in particular when performing tests on children, but that can give rise to other non-negligible drawbacks.
There also exist implanted devices for picking up auditory evoked potentials such as those described in patent U.S. Pat. No. 5,758,651 or application WO 97/48447, which measure cochlear activity following electrical stimulation thereof.
Document U.S. Pat. No. 5,758,651 describes a telemetry system for a hearing prosthesis, in particular for a cochlear implant. The system has a plurality of electrodes which are used to stimulate the auditory nerve and to detect the evoked potentials; in an example described in that document, the potential difference between an intracochlear electrode and an extracochlear electrode is measured; that potential difference is applied to the input of a “blanking” amplifier of gain adjustable over the range 40 dB to 80 dB; in variant, that document proposes using a differential amplifier.
Document WO 97/48447 describes an adjustment system integrates in a cochlear stimulation implant; the system uses two means for measuring auditory perception by the patient: firstly an intracochlear electrode for measuring auditory perception by the patient: firstly an intracochlear electrode for measuring evoked potential, and secondly an electrode (or transducer) for measuring the activity of one of the two muscles of the middle ear (“stapedius” and “tensor tympani”).
The document by Caroline J. Brown et al. Published in J. Acous T Soc AM, Vol. 88, No. 3, September 1990, pages 1385 1391, describes a method of measuring evoked potentials in which use is made firstly of one of six intracochlear electrodes of the stimulation implant, an d secondly of an electrode placed on the temporal muscle.
Those devices pick up solely a near field auditory evoked potential because one of the pickup electrodes is immersed in the cochlea and is as close as possible to the activity to be measured. That near field potential which represents the initial excitation of the auditory system is commonly referred to as evoked action potential (EAP). It does not give information about the other levels of the auditory system: cochlear nucleus, coliculus inferior, primary and secondary auditory areas, associative areas. Furthermore, EAP cannot be recorded directly because of artifacts generated by electrical stimulation too close the pickup electrodes. In order to obtain a meaningful EAP, it is necessary to make use of so-called “subtractive” techniques that serve to remove the artifacts and allow the EAP to be extracted on its own. Both techniques are described ion particular in the above-mentioned document By Carolyn J. Brown et al.